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TV and radio towers kill nearly 7 million birds every year in North America, according to a recent study.

There are 84,000 communication towers on the continent, and those higher than 1,000 feet present the greatest danger, according to the study by Travis Longcore at University of Southern California. The birds often run into the cables that hold the towers up.

Building the towers at a lower altitude would save million of these birds every year, Longcore said:

The taller the tower the greater the threat, the study found. The 1,000 or so towers above 900 feet accounted for only 1.6 percent of the total number of towers. Yet these skyscraper towers killed 70 percent of the birds, about 4.5 million a year.

The study focused on the taller towers used for TV and radio frequencies. Cell phone towers are shorter and aren't seen as a threat.